The invention relates to a method of adjusting print uniformity in a xerographic device. It finds particular application in conjunction with adjusting print uniformity by adjusting individual light emitting diode (LEDs) in an LED printbar of a xerographic device and will be described with particular reference thereto. However, it is to be appreciated that the invention is also amenable to other applications.
Full width array imagers that are used in image recording systems are well known in the art. Such imagers are generally comprised of a linear array of discrete sources. The sources may emit ink, ions, or light. Examples of full width array imagers include wire dot, electrostatic, ink jet, and thermal print heads. Light emitting diode (LED) full width array imagers are commonly used because of their high resolution and fast response time. They consist of an arrangement of a large number of closely spaced LEDs in a linear array. By providing relative motion between the LED printbar and a photoreceptor, and by selectively energizing the LEDs at the proper times, a desired latent electrostatic image can be produced on the recording member. The production of a desired latent image is usually performed by having each LED expose a corresponding pixel on the recording member in accordance with image-defining video data information applied to the printbar through driver circuitry. Conventionally, digital data signals from a data source, which may be a Raster Input Scanner (RIS), a computer, a word processor or some other source of digitized image data is clocked into a shift register. Some time after the start of a line signal, individual LED drive circuits are then selectively energized to control the on/off timing of currents flowing through the LEDs. The LEDs selectively turn on and off at fixed intervals to form a line exposure pattern on the surface of the photoreceptor. A complete image is formed by successive line exposures.
The light emitted by each element in an LED printbar is controlled by the element""s input current. Due to manufacturing variations, actual LED printbars will not show equal light outputs when the input currents for each LED are the same. These variations in light output expose the photoreceptor differently and give undesirable streaks in the prints (in the process or slow-scan direction). Currently, to make the emitted lights uniform across the bar, the light from each element is measured with a single photodiode that is moved along the bar; and the input currents to individual elements are adjusted until the emitted lights are equalized. There are two important drawbacks associated to this technique. First, the adjustment cannot be done with the LED printbar mounted in the printer; and thus the procedure takes a lot of time. Second, the uniformity of the emitted light does not guarantee the uniformity of the print on a target media since this uniformity depends also on metrics of the beam shape and xerographic effects. A narrow beam and a broad beam with the same total power will give rise to spots or lines of different width. Line scan cameras can map the beam shape. Still, the effect xerography will have on different beams depends on the xerographic setpoints and the state of the material cannot be predicted with certainty.
Furthermore, the LEDs may have different aging characteristics that will change the pixel-to-pixel non-uniformity. To a first approximation, a decrease in an individual LED""s light output is a simple function of the LED""s accumulated xe2x80x9conxe2x80x9d time. This xe2x80x9conxe2x80x9d time will vary for each individual pixel based on the history of images printed by the printer. A prior art solution to the aging problem is to periodically measure the light output as each pixel is individually turned on. When degradation is detected, the outputs for the degraded LEDs are adjusted. However, again this technique does not give streak free prints for the same reasons described above.
Additionally, U.S. Pat. No. 5,668,587 discloses a technique for adjustment of LED current for LEDs in a printbar based on accumulated counts representative of LED xe2x80x9conxe2x80x9d time. The technique achieves uniformity in light output of the LED printbar in a manner that senses the operational xe2x80x9conxe2x80x9d time of each LED. When the xe2x80x9conexe2x80x9d time difference between the LED with the longest xe2x80x9conxe2x80x9d time and the LED with the shortest xe2x80x9conxe2x80x9d time exceeds a predetermined value, the outputs of at least some LEDs comprising the printbar are compensated to normalize their light output. This normalization is achieved via a drive circuit that drives each LED based upon correction data, and by determining an update of the correction data based upon the difference between the xe2x80x9conxe2x80x9d time of individual LEDs. The correction data is determined when the difference between the xe2x80x9conxe2x80x9d time of the LED with the longest xe2x80x9conxe2x80x9d time and the xe2x80x9conxe2x80x9d time of the LED with the shortest xe2x80x9conxe2x80x9d time exceeds a predetermined maximum difference.
Furthermore, U.S. Pat. No. 5,859,658 discloses a technique for adjustment of LED current for LEDs in a printbar based on changes in the I-V characteristics of the LED. The technique is accomplished using an apparatus for compensating LED printbars, and printers that use LED printbars, for aging. Changes in the slopes of the forward voltage drop verses forward current characteristics (the I-V characteristics) of the LEDs of the LED printbar are determined. The drive currents of the LEDs are then changed as a function of the changes in the slopes so as to compensate for changes in the LED light outputs.
Thus, there is a particular need for a method of adjusting print uniformity by adjusting individual LEDs in an LED printbar of a xerographic device. The invention contemplates several methods of adjusting print uniformity that overcome at least one of the above-mentioned problems and others.
In one aspect of the invention, a method of adjusting print uniformity for a xerographic device having an LED printbar is provided. The method includes: a) printing a test pattern line on a target media in the process direction of the xerographic device, wherein the test pattern line is associated with an individual LED of the LED printbar; b) transferring the target media with the printed test pattern line to a scanner, c) scanning the target media and detecting the printed test pattern line; d) communicating detected test pattern line information to a computer; e) determining a measured metric from the detected test pattern line information; f) calculating the difference between the measured metric and a target value; and g) if the absolute value of the difference exceeds a first predetermined threshold, adjusting the current supplied to the individual LED associated with the test pattern line to reduce the difference. In another embodiment, the steps (a) to (g) may be iterated two or more times until the absolute value of the difference is below a predetermined threshold.
In another aspect of the invention, another method of adjusting print uniformity for a xerographic device having an LED printbar is provided. This method works for high resolution LED imagers, where process direction lines printed with a single LED illuminated are too narrow to print out. The method consists of grouping the LED""s by threes, and printing out different combinations of each group. Specifically, the method includes: a) printing a first test pattern line, a second test pattern line, and a third test pattern line on a target media, wherein each test pattern line is in the process direction of the xerographic device, wherein the first test pattern line is associated with a first LED of the LED printbar and an adjacent second LED, wherein the second test pattern line is associated with the second LED and an adjacent third LED, wherein the third test pattern line is associated with the first, second, and third LEDs, and wherein each test pattern line is spaced from adjacent test pattern lines; b) transferring the target media with the printed first, second, and third test pattern lines to a scanner; c) scanning the target media and detecting the printed first, second, and third test pattern lines; d) communicating detected first test pattern line information, detected second test pattern line information, and detected third test pattern line information to a computer associated with control of current supplied to individual LEDs of the LED printbar; e) determining a first measured metric from the detected first test pattern line information, a second measured metric from the detected second test pattern line information, and a third measured metric from the detected third test pattern line information; f) calculating the difference between the first measured metric and a first target value; g) calculating the difference between the second measured metric and the first target value; h) calculating the difference between the third measured metric and a second target value; and i) if the absolute value of the difference in one or more of steps f) through h) exceeds a predetermined threshold, isolating the difference to one of the first, second, or third LEDs and adjusting the current supplied to the isolated LED to reduce the difference.
In yet another aspect of the invention, a xerographic device is provided. The xerographic device including an LED printbar having a plurality of individual LEDs, a computer, and a scanner device.
Benefits and advantages of the invention will become apparent to those of ordinary skill in the art upon reading and understanding the description of the invention provided herein.